Metal-gas cells or batteries, such as nickel-hydrogen cells, are contained in sealed metal vessels or casings (often referred to as cans) which contain hydrogen gas under high pressure. This gas pressure may, for example, vary between about 0 psi and 1000 psi during each cycle charge-discharge operation. Each such cell has at least one nickel-containing positive electrode which is spaced from a hydrogen-forming negative electrode. A pair of positive and negative electrodes makes up the individual cell, with a plurality of such cells forming the battery. A plurality of such electrode pairs are generally organized in the form of plates which are stacked to form a plate stack. The stack also includes gas diffusion plate separators which prevent short circuiting contact between the positive and negative electrodes, and which also hold a sufficient quantity of electrolyte for cell operation.
The electrolyte is typically an alkaline medium such as an aqueous solution of alkali metal hydroxide, generally approximately a 30% potassium hydroxide solution. The negative (hydrogen-forming) electrode or anode is typically a plastic bonded, metal powder plate. The metal powder is usually platinum or palladium which will catalyze a hydrogen dissociation reaction in the aqueous electrolyte. The plastic bonding material can be a tetrafluoroethylene, for example. The active material of the positive plate or cathode is generally nickeloxyhydroxide.
Hydrogen in the vessel diffuses through a diffusion mesh of tetrafluoroethylene or the like to reach the catalytic anode. The anode causes molecular H.sub.2 to dissociate into atomic hydrogen, which in turn reacts with free hydroxyl groups to form water plus free electrons. The water and the free electrons react with the nickeloxyhydroxide cathode to form nickel hydroxide plus free hydroxyl groups. Reverse reactions occur during charging.
The components of the cell stack (i.e., anodes, cathodes, separators) are conventionally made in a disc-shape and are arranged along a common axis. A single plate stack has been used, which is mounted PG,4 in a cylindrical shaped pressure vessel having hemispherical ends. The pressure vessel is ordinarily formed from two casing portions joined at an equatorial weld ring.
Nickel-hydrogen cells are relatively long lived, have a wide operating temperature range and a high energy density. They have been widely adopted as a preferred electrical storage system for earth-orbiting satellites.
Due to the great expense of these satellites, the chance of cell failure must be absolutely minimized. The cells must as well be designed to endure the forces encountered when the satellite is launched, for example. It is also most critical that the mass and volume of these cells be as low as possible, while their energy storage and generation capabilities are maximized. For these reasons, considerable attention has been directed to the structure which supports the cell stack in the pressure vessel.
One known way to support the plate stack is by mounting it on a retaining rod which extends axially through a central aperture formed in the components of the stack. The retaining rod is supported at its ends by terminals which extend axially outwardly from the centers of the domed ends of the pressure vessel. The plate stack is fixed and may be compressed on the rod by stops or retaining elements.
Mounting the plate stack on a central rod in this manner leads to a relatively large mass and volume for the nickel-hydrogen cell, and further places an undesirable stress on the terminals themselves. This makes the cell more prone to failure. It also makes the cell relatively long due to the axially extending terminals.
One previously known alternative to mounting the plate stack on a central rod is to mount or support the stack in a cantilever fashion from a support surface formed inside the pressure vessel, such as from one side of the weld ring used in interconnecting the two domed casing portions. In this arrangement, the weld ring has been located considerably to one axial side of the center or middle of the pressure vessel. The plate stack is cantilevered (as viewed horizontally) from one side of the weld ring, as on an elongated support rod fixed to the weld ring, and extends through much of the remaining volume of the pressure vessel.
One difficulty with this type of mount for the plate stack is the relatively long lever arm represented by the plate stack on the support rod. Jolting of the cell from acceleration and deceleration or other movements causes the relatively heavy plate stack to vibrate off-axis, particularly at its free end, imposing a torque on the weld ring. Such movements can also cause the stack to bang against the inside of the pressure vessel. Failure of the cell can result, such as from damage to the components of the cell stack from banging into the pressure vessel walls, shorting of the electrodes through contact with the metal pressure vessel, or rupture of the weld ring seal from fatigue.
One possible solution to this problem is to reduce the number of individual cells making up the cell stack, to thereby shorten the lever arm. This reduces the capacity of the battery, however.
Another drawback to cantilever mounting of the cell stack in this fashion is inherent in the fabrication of the pressure vessel itself. The draw process which is typically used for making the pressure vessel limits the maximum size of a can portion to approximately 5-7" in axial length for a 31/2" diameter cell. (A 31/2" diameter cell is at present standard in the industry for satellite applications.) A pressure vessel is ordinarily constructed with one can portion of about this maximum axial length and within which substantially all of the cell stack is received, and a shorter can portion (or cap) completing the vessel. The maximum length of the plate stack is thus limited by the maximum length of the larger can portion. This construction further reduces the volume available for mounting the plate stack within the pressure vessel, again limiting the capacity of the cell.
Another problem with mounting the plate stack from one side of a weld ring in the foregoing fashion is that heat transfer between the plate stack and the pressure vessel is not efficient. The point of highest heat generation (i.e., during discharge) in a plate stack is at the longitudinal center of the stack. In the cantilever arrangement described above, the center of the stack is considerably offset from the weld ring which transfers heat to the pressure vessel for dissipation.
Also, the larger can portion containing the plate stack has a tendency to balloon or bulge somewhat from the internal pressure, particularly if this larger portion is made to maximum length. This causes undesirable flexing of the pressure vessel during internal pressure changes associated with cyclic charge-discharge operation. It also increases the gap between the plate stack and the pressure vessel wall within which the plate stack can vibrate or shift, again causing greater torque on the weld ring.